High Coercivity Nd-Fe-B Sintered Magnets With Low Cost

Sintered Nd-Fe-B magnets have been widely used in various fields owing to their outstanding magnetic properties and high cost performance. Currently, the energy products of Nd-Fe-B magnets have already been close to the theoretical value while the coecivity is less than one third of the theoretical value. The low coercivity severely limits their applications in high operating temperature. Therefore, how to improve coercivity through opertimiazing the composition and microstructure has became one of the research hot issues. Based on the needs of industrial application, low-cost fabrication techniques of high coercivity Nd-Fe-B magnets were investigated in this work. We prepared high coercivity Nd-Fe-B magnets through intergranullar addition and dual main alloy method. The effects of cheaped Dy2O3powders and heavy rare earth Dy addition on magnetic properties and microstructure of Nd-Fe-B were investigated.Results indicated that the coercivity significantly increased with the intergranular addition of Dy2O3while the remanence and energy products maintained due to the suppressed magnetic dilution. For Dy-free magnets, the coercivity was23.0kOe when added6.0wt%Dy2O3, while the remanence quickly decreased to11.6kGs. For2.0wt%Dy-containing magnets, the coercivity was enhanced gradually from17.5kOe to21.7kOe when added with2.0wt%Dy2O3and further to24.3kOe with4.0wt%addition, while the ramanence can be maintained above12.0kGs even adding4.0wt%Dy2O3. Microstructure investigations showed that Dy diffused towards the grain surface layers of the2:14:1matrix phase improving the magnetocrystalline anisotropy. Meanwhile, the addition of high melting point Dy2O3suppressed the grains abnormal growth and optimized the distribution of the intergranular phase, which were also essential to enhance the coercivity.Through dual main alloy method, we prepared series high-performance magnets with Hcj (kOe)+(BH) max (MGOe)>62, such as N52, N48M or N45H. Investigations showed that Dy diffused from high HA matrix phases to low HA phases forming (Nd, Dy)2Fe14B phase with high magnetocrystalline anisotropy in the liquid phase sintering process, which was the main factor of the increasing coercivity. The grain boundary phase became continuous and uniform with the addition of high HA phases, which was also essential to enhance the coercivity.